1. Field of the Invention
The present invention relates to a power controllable wireless mobile communications system employing an adaptive modulation and coding scheme and a method therefor. More particularly, the present invention relates to an apparatus that controls output power of a mobile station in a mobile communications system employing an adaptive modulation and coding scheme by using a signal-to-noise ratio (SNR) extracted from a backward signal that is transmitted from the mobile station to a base station and a method therefor.
2. Description of the Related Art
In general, the ultimate aim of power control in a wireless communications system is to provide a predetermined quality to communications channels by compensating for signal attenuation in wireless channels such that a signal is received at an appropriate signal level requested by a receiving end. Also, as wireless communications networks have evolved to cellular networks, power control has become an important factor in system performance and system capacity due to co-channel interference or multi-user interference. Particularly, in a code division multiple access (CDMA)-type mobile communications system, the capacity of the system is determined by the signal-to-noise ratio (SNR) at a receiving end. If a requested SNR (SNR_REQ), which is a predetermined signal level requested by a demodulation end, is maintained, the number of simultaneous users on the system may be increased. Accordingly, if a near-far problem in the backward direction is minimized by properly adjusting each user's power control, a frequency efficiency that is higher than that of other transmission methods may be attained.
An example of a power control method in a conventional mobile communications systems is a fixed step-size closed-loop power control (FCLPC) method recommended by Interim Standard (IS)-95. In the FCLPC method, a base station measures the strength of a signal received from a mobile station (or a terminal) and then transmits a power control command to the mobile station. If the strength of the received signal is lower than a requested SNR, the base station sets a power control bit contained in the power control command to ‘0’ so that the transmission signal strength of the mobile station is increased by 1 dB. If the strength of the signal is higher than the requested SNR, the base station sets the power control bit to ‘1’ so that the transmission signal strength is decreased by 1 dB. That is, in this method the scope of changes in the transmission power of the mobile station is fixed at ±1 dB. While such a technique may be effective for a situation where the signal is changed slowly and gradually, it is not appropriate where the signal is widely or rapidly changing because this method has a limited converging speed.
Recently, in order to provide high speed transmission rates, modulation methods having high frequency efficiency have been employed. However, if channel states are poor, the modulation methods having high frequency efficiency cannot be used. Therefore, an adaptive modulation and coding (AMC) scheme is used in which a modulation method and channel coding scheme can be adaptively selected according to the state of channels. In a system employing the AMC scheme, the SNR requested by a receiving end varies according to a threshold scheme that may graphically be represented in the shape of a staircase.
FIG. 1 illustrates a graph showing changes of requested SNR with respect to time in a mobile communications system employing the adaptive modulation and coding (AMC) scheme and changes of SNR with respect to time according to a conventional power control method. In the graph, the smooth curve indicates the requested SNR when the modulation method and coding changes from 64 quadrature amplitude modulation (QAM)+¾ to quadrature phase-shift keying (QPSK)+¾ according to the AMC scheme and the staircase-shape line indicates the SNR using a conventional power control method. Specifically, the graph shows that the conventional method cannot satisfy the requested SNR in real time.
That is, the conventional power control method does not consider the AMC scheme where the SNR changes rapidly in each packet or frame. Accordingly, the conventional power control method cannot provide real-time control and may cause cutoffs during calls due to an unstable receiver signal, and may also cause adverse side effects such as decreasing the number of simultaneous users capable of being on the system due to inefficient use and waste of signal transmission power. In addition, if all mobile stations in a cell or in a network exhibited the result shown in FIG. 1, a decrease in channel capacity may occur.
To solve the above problem, methods for variable power control with respect to changes in the level of a signal based on the moving speed of a mobile station have been proposed. However, these methods consider the changing scope of a signal with respect to the moving speed of a mobile station, assuming that an identical modulation and demodulation method is employed. Accordingly, these methods are not optimized for the AMC scheme in which modulation and demodulation schemes change. Therefore, a power control method appropriate to a system employing the AMC scheme is needed.